In many radio receiver applications it is important to be able to accurately tune a resonant circuit without relying on a signal strength as an indication of the tuning accuracy. This capability is particularly important in such applications as automatic direction finders for aircraft.
For navigational purposes many areas have a plurality of radio transmitting homing beacons located at known points and transmitting at predetermined frequencies. Each of these stations in any particular locality operates at a different frequency. A pilot, in navigating his aircraft may well wish to set his direction finder to the frequency of a beacon he is approaching. However, this act may occur at a time when the received signal is very weak or is not received at all. Furthermore, navigating an aircraft is a very demanding task and the pilot normally has many other duties to see to, other than refining the tuning of his direction finder.
For these and other reasons a desirable goal has been to provide an accurate tuning device which is accurately tunable to selectable different frequencies in the presence of a weak signal or no signal at all. A number of prior art tuners have been arranged to vary the tuning frequency in digital increments, and this is found to be quite useful and appropriate.
As reported by Ertman, in U.S. Pat. No. 3,244,983, conventional analog-type tuning with ganged tuning condensers in the RF and local oscillators cannot select, for example, a 1 kc signal in a 30 megacycle range. Tuning dials and condensers cannot be calibrated to accurately obtain such resolution even if the dials could be read by the operator.
Although the drawbacks in ganged tuning condensers are well known the solutions offered by Ertman in the afore-mentioned patent and by Bean in his U.S. Pat. No. 3,681,707 are exceedingly complex and thus expensive.